Electron irradiation apparatus



May 19, 1959 J. C. NYGARD ELECTRON IRRADIATION APPARATUS Filed 001:. 1, 1957 United States Patent ELECTRON IRRADIATION APPARATUS John C. Nygard, Lexington, Mass., assignor to High Voltage Engineering Corporation, Burlington, Mass., a corporation of Massachusetts Application October 1, 1957, Serial No. 687,632

2 Claims. (Cl. 250-495) This invention relates to the irradiation of materials with electrons which have been accelerated to high energy by means of an electron accelerator, from which the electrons emerge at high energy through an electron window, and in particular to apparatus for filling the space between the electron window and the product with an inert gas. High energy electron beams are being increasingly used for the irradiation of various materials in radiation chemistry and in the sterilization or preservation of food and drugs. The beam of electrons is commonly produced by accelerating electrons to high energy in an evacuated region. After being accelerated to the required energy the electrons emerge from the evacuated region through an electron window comprising a thin metal foil and are directed onto the product to be irradiated. The electron window is made as thin as possible in order to minimize absorption of electron energy in the window. However, the electron window must have sufiicient mechanical strength to support atmospheric pressure against the vacuum within the evacuated region. Among the other undesirable aspects of fire or explosion during irradiation is the possibility that the window may break in view of its delicate structure. When the electron window breaks, the atmosphere rushes into the acceleration tube and, primarily because of its oxygen content, causes substantial damage within the acceleration tube.

Very little electron energy is lost in the acceleration tube itself because of the high vacuum existing therein. However, in addition to energy losses in the electron window hereinbefore mentioned, a certain amount of energy is lost due to scattering in the air intervening between the electron window and the product. This scattering increases with increasing atomic number of the scattering gas.

Other hazards associated with electron irradiation include fire and ozone production both of which are associated with the presence of oxygen in the irradiation area. Since irradiation involves delivery of energy to the product, its temperature is raised and the possible danger of fire exists. Moreover, the effect of the irradiation on the oxygen in the air intervening between the electron window and the product is to produce ozone which can damage not only the product but also electronic equipment in the immediate vicinity, such as beam scanning equipment which is used on many accelerators.

The invention may best be understood from the followin; detailed description thereof having reference to the 2 accompanying drawing in which the single figure illustrates one embodiment of the invention.

Referring to the single figure of the drawing, electrons are accelerated to high energy within an evacuated electron acceleration tube, the lower extremity of which is shown at 1, by any conventional electron accelerating means, such as an electrostatic accelerator, a resonance transformer or a microwave linear accelerator. After being accelerated, the electrons emerge through an electron-permeable window 2 in the form of a beam 3.

The product 4 to be irradiated is conveyed through the electron beam 3 upon a conveyor belt 5. In accordance with the invention, a hood 6 is hermetically sealed to the lower extremity 1 of the electron accelerator, and an inert gas, such as helium, argon, etc., is introduced under said hood 6 through a suitable conduit 7. Since a gas such as helium is lighter than air, it is retained under the hood 6, and the product conveyor is so arranged that the product is elevated during irradiation so as to be in the inert gas atmosphere. For example, a series of pulleys 8 may be provided to guide the conveyor belt 5 up under the hood 6.

In this manner the space between the electron window 2 and the product 4 is filled with a gas of low atomic number, such as helium; and therefore electron scattering is minimized. Moreover, since the atmosphere in which the ionization is produced by the electron radiation is inert, the danger of fire under the electron window 2 is minimized. Moreover, in the event of puncture of the electron window 2, the danger of damage to the acceleration tube is minimized, since the acceleration tube will be filled not with air but with the inert gas. Moreover, in accordance with the invention, ozone production and damage to the product or to electronic equipment associated with the electron accelerator resulting therefrom is minimized.

Having thus described the principles of the invention, together with an illustrative embodiment thereof, it is to be understood that although specific terms are employed they are used in a genetic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

1. Apparatus for electron irradiation comprising in combination with an electron accelerator having an electron window, a chamber substantially enclosing the space outside said electron window into which electrons issue from said electron window, but having an aperture in its lowermost surface, means for introducing an inert gas which is lighter than air into said chamber, and means for conveying the product to be irradiated into the path of the electron beam within the inert gas atmosphere.

2. Apparatus in accordance with claim 1 wherein said inert gas includes helium.

References Cited in the file of this patent UNITED STATES PATENTS 

